I. Introduction
The absorption cycle is one approach to heat-actuated cooling and is similar to a vapor compression cycle cooler except that the mechanical compressor in the vapor compression cycle is replaced with a chemical compressor. Chemical compressors typically have five components: an absorber, a desorber, an expansion valve, a regenerative heat exchanger and a pump. In the desorber, a mixture of fluids is heated and the refrigerant is desorbed from the mixture as a vapor. The refrigerant is at a high pressure and is condensed, rejecting heat to the surroundings. The refrigerant then passes through the expansion valve, where the pressure is reduced and the refrigerant boils at a low temperature. Heat is taken from the load and used to boil the refrigerant to form refrigerant vapor. The refrigerant vapor is then absorbed into the circulating fluid. The mixture is pressurized by a pump and returned to the desorber. Because the mixture is an incompressible liquid when it is pressurized, the pump work typically is 1/100 of the electric power required to operate a similar mechanical compressor. Heat is added to the system in the desorber and heat is rejected from the system in the condenser and absorber. A schematic diagram of an absorption heat pump is shown in FIG. 1.
The absorption cycle may be classified by fluid combination and cycle arrangement. The most widely used fluid combinations are (1) lithium bromide (LiBr) and water (H2O), where water is the refrigerant, and (2) water and ammonia, where ammonia is the refrigerant. Typically, LiBr/H2O systems are used where low temperature (<150° C.) thermal energy is available to drive the cycle. Ammonia and water systems are used for heating applications where the system is rejecting heat to surroundings that can be at a high temperature (such as automotive cooling) and applications where high efficiency is critical. In a water and ammonia absorption heat pump, the desorber is operated at a sufficiently high pressure to insure that the temperature of the water is below the saturation temperature; consequently, the water will not boil during the desorption process.
Macroscale absorption heat pumps using falling films are commercially available and are widely used for stationary cooling applications, particularly where waste heat is available. The U.S. Department of Energy has sponsored a large-scale, multi-year research program focused on improving the performance of heat-actuated heat pumps. As part of these development efforts significant research has been focused on improving the performance of falling films absorbers and desorbers.
Gas/liquid contacting (absorption or desorption) is critical for absorption-based, heat-actuated heat pumps. However, for known systems that use thin liquid films, the size of the gas/liquid contactor is fundamentally limited by diffusion in the liquid film where the diffusivity in the gas phase can be three to five orders of magnitude greater then in the liquid phase. Falling films are inherently orientation dependent and typically result in liquid film thicknesses on the order of 1 mm. Reducing the thickness of the liquid film in a gas/liquid contactor from 1 mm to 100 microns would increase mass transfer by a factor of about 100. When the resistance to mass transfer in gas desorption and absorption is dominated by diffusion in the liquid phase the use of extremely thin films can radically reduce the size of a gas desorber or absorber.
But, using a falling film results in an absorption heat pump that is orientation sensitive. A small deviation from the design orientation prevents the falling film from properly forming. As a result, the heat pump does not function properly.
Recent developments suggest that the performance of absorbers and desorbers in a heat-actuated heat pump can be significantly enhanced by using a membrane with many small diameter pores to prevent mixing between a gas and liquid while allowing the gas and liquid to be in contact. When the contactor is properly designed, surface tension prevents the liquid from entering the pores while the pores provide a path for gas to diffuse to the liquid surface. The contactor then can be used to form a film with a thickness of less than 100 microns while allowing gas/liquid contact.
While there has been significant progress in the development of a lithium bromide and water absorption cooler for manportable military applications, most civilian applications require the use of ammonia and water. Lithium bromide systems cannot be used for space heating (the refrigerant is water and would freeze when operating at low temperature) or automotive cooling where the high heat rejection temperature prevents the use of lithium bromide and water. The desorber in an ammonia and water system operates at a temperature (≈250° C.) that is sufficiently high to prevent the use of membranes. Therefore, an alternative compact desorption scheme is needed before microtechnology-based, heat-actuated heat pumps can be successfully applied to applications that require ammonia and water for the heat pump working fluids.
Aqueous lithium bromide and ammonia-based heat pumps or refrigeration systems that use thin liquid films are known in the patent literature. For example, U.S. Pat. No. 5,811,062 describes embodiments of a micro-device useful for conducting chemical separations and conversions, including any exchange of a compound or element from one solvent to another where the solvents may be liquid or gas or both, e.g., an absorption cycle refrigeration system. Example 3 of the '062 patent concerns gas absorption into a liquid, e.g., ammonia vapor absorbed into liquid water. Ammonia was absorbed into the water within the liquid film on the microporous contactor. The ammonia flow rate varied from 0-4 grams/minute with water flow rate ranging from 0-33 grams/minute.
Several alternatives to falling films also have been investigated. These alternatives include desorption from liquid films on spinning disks, and mechanically constrained thin films. Spinning disks have been shown to significantly reduce liquid film thickness, but this approach also is orientation dependent. When compared to conventional falling film absorption and desorption, the mechanically-constrained, ultra-thin-film technology has the potential for a striking improvement in performance. With a mechanically-constrained, ultra-thin film of water preliminary results suggest that ammonia can be absorbed in water at a rate that will produce between 10 and 30 W/cm2 of thermal energy (the heat of condensation in the absorption process). This is an extraordinary absorption rate that exceeds the performance of conventional absorbers by more than a factor of 10. This level of performance offers a ten-fold reduction in the size of a conventional absorption heat pump.
II. Microchannel Arrays Made by Laminate Architecture
Microchannel arrays useful for practicing aspects of embodiments disclosed by the present application can be made using laminate architecture. Methods for making microchannel arrays using laminate architecture are described in Paul et al. 's U.S. Pat. Nos. 6,672,502 and 6,793,831, which are incorporated herein by reference.
III. Fractal Plates
Fractal plates also are described in applicant's pending U.S. Pat. No. 6,688,381, which also is incorporated herein by reference.